The aim of this research is to develop and apply the techniques of combined gas chromatography/mass spectrometry to the study of the fragmentation, metabolism, distribution and excretion of nucleosides of biochemical and clinical interest. The successful attainment of these objectives requires the synthesis of stable isotopically labeled nucleosides, an understanding of the fragmentation of these molecules, the development of techniques for the separation and identification of specific molecules in a complex mixture of compounds isolated from a biological matrix and the quantitation of nucleosides, and their metabolities, of clinical importance as anticancer agents isolated from biological samples. Application of these techniques to studies involving subjects is the ultimate goal of this work and efforts are being made to establish the necessary collaborative agreements. The results of this work would be of significant value in designing dosage schedules for the administration of antitumor agents and in the design of nucleoside analogs with increased antitumor activity. These techniques may also be of significance in the diagnosis of cancer and in following the efficacy of the treatment of cancer by monitoring the urinary excretion of modified nucleosides by cancer patients both before and after therapy.